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Crystallochemical Design of Huntite-Family Compounds

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Crystallochemical Design of Huntite-Family Compounds crystals Review Crystallochemical Design of Huntite-Family Compounds Galina M. Kuz’micheva 1, Irina A. Kaurova 1,* , Victor B. Rybakov 2 and Vadim V. Podbel’skiy 3 1 MIREA-Russian Technological University, Vernadskogo pr. 78, Moscow 119454, Russia; [email protected] 2 Lomonosov Moscow State University named M.V. Lomonosov, Vorobyovy Gory, Moscow 119992, Russia; [email protected] 3 National Research University Higher School of Economics, Myasnitskaya str. 20, Moscow 101000, Russia; [email protected] * Correspondence: [email protected]; Tel.: +7-495-246-0555 (ext. 434) Received: 18 December 2018; Accepted: 12 February 2019; Published: 15 February 2019 Abstract: Huntite-family nominally-pure and activated/co-activated LnM3(BO3)4 (Ln = La–Lu, Y; M = Al, Fe, Cr, Ga, Sc) compounds and their-based solid solutions are promising materials for lasers, nonlinear optics, spintronics, and photonics, which are characterized by multifunctional properties depending on a composition and crystal structure. The purpose of the work is to establish stability regions for the rare-earth orthoborates in crystallochemical coordinates (sizes of Ln and M ions) based on their real compositions and space symmetry depending on thermodynamic, kinetic, and crystallochemical factors. The use of diffraction structural techniques to study single crystals with a detailed analysis of diffraction patterns, refinement of crystallographic site occupancies (real composition), and determination of structure–composition correlations is the most efficient and effective option to achieve the purpose. This approach is applied and shown primarily for the rare-earth scandium borates having interesting structural features compared with the other orthoborates. Visualization of structures allowed to establish features of formation of phases with different compositions, to classify and systematize huntite-family compounds using crystallochemical concepts (structure and superstructure, ordering and disordering, isostructural and isotype compounds) and phenomena (isomorphism, morphotropism, polymorphism, polytypism). Particular attention is paid to methods and conditions for crystal growth, affecting a crystal real composition and symmetry. A critical analysis of literature data made it possible to formulate unsolved problems in materials science of rare-earth orthoborates, mainly scandium borates, which are distinguished by an ability to form internal and substitutional (Ln and Sc atoms), unlimited and limited solid solutions depending on the geometric factor. Keywords: huntite family; rare-earth scandium borate; optical material; crystal growth; rare-earth cations; X-ray diffraction (XRD); crystal structure; solid solution; order–disorder 1. Introduction Modern scientific and applied materials science requires both an appearance of new materials with a desired combination of functional properties and an improvement and optimization of physical parameters and structural quality of the known materials, which have already proven themselves in practice, with further control of their properties using external (growth and post-growth treatment conditions) or internal (activation, isomorphic substitution) effects. Isomorphic substitutions and activation are different in the concentration of ion(s) introduced into a crystal matrix and effects produced. Isomorphic substitution is a powerful and flexible way to obtain a desired physical parameter of material by a targeted change in the composition of specific crystal structure. Activation is one of the necessary and relatively simple technological actions for the appearance, modification, and improvement of crystal properties by introduction (sometimes, over stoichiometry) of small amounts Crystals 2019, 9, 100; doi:10.3390/cryst9020100 www.mdpi.com/journal/crystals Crystals 2019, 9, 100 2 of 49 of impurity ions into a crystal matrix that contributes to a self-organization and self-compensation of system’s electroneutrality. It is possible that dopant ions introduced into a crystal structure in low concentrations over stoichiometry are distributed over crystallographic sites in a different way than those introduced in high concentrations in the case of formation of solid solutions. However, low content of dopant ions can lead to significant changes in both local and statistical structures, in particular, in crystal symmetry. In this paper, all the above-mentioned aspects are reviewed for the huntite family compounds, interesting and important materials from both applied and scientific points of view, with the main focus on single-crystal objects, a detailed study of which can obtainin reliable information about the internal structure of materials. Complex orthoborates of rare-earth metals with the general chemical formula LnM3(BO3)4, where 3+ 3+ Ln = La–Lu, Y and M = Al, Ga, Sc, Cr, Fe, belong to the huntite family (huntite CaMg3(CO3)4, space group R32 [1]). Depending on the composition and external conditions, they can have both monoclinic (space groups C2/c, Cc, and C2) and trigonal (space groups R32, P321, and P312) symmetry with the presence (space group C2/c) or absence of center of symmetry. The most widely known and studied representatives of the huntite family compounds are 3+ 3+ aluminum borates LnM3(BO3)4 with the M = Al. The LnAl3(BO3)4 crystals with the Ln = Y, Gd, Lu doped with the Nd, Dy, Er, Yb, Tm (for example, [2,3]) and co-doped with the Er/Tb, Er/Yb, Nd/Yb ions (for example, [4,5]), as well as single-crystal solid solutions in the YAl3(BO3)4 – NdAl3(BO3)4 system [6], are promising materials for self-frequency-doubled lasers. YAl3(BO3)4 crystals doped with the Er3+ or Yb3+ ions are widely used in medicine and telecommunications as laser materials 3+ with a wavelength of 1.5–1.6 µm [4,5]. The LnAl3(BO3)4 crystals with the Ln = Tb, Ho, Er, Tm exhibit a magnetoelectric effect (HoAl3(BO3)4 is a leader among these compounds) (for example, [2,7]). 3+ The LnAl3(BO3)4 compounds with the Ln = Gd, Eu, Tb, Ho, Pr, Sm are used as phosphors (for 3+ 3+ example, [8,9]): the YAl3(BO3)4 crystals doped with the Eu and Tb ions is an environmentally friendly material for a white LED, having high intensity and luminescence power and low cost [10]; those doped with the Tm3+ and Dy3+ are able to be tuned from blue through white and ultimately to yellow emission colors and are attractive candidates for general illumination [11]; those doped 3+ 3+ with the Sm ions, as well as GdAl3(BO3)4:Sm , can be used as promising materials for orange-red lasers [8]. The simultaneous generation of three basal red–green–blue colors is obtained from a lone 3+ GdAl3(BO3)4:Nd bi-functional laser and optical nonlinear crystal [12], which is also of interest for the development of high-quality and bright displays. 3+ Rare-earth gallium borates LnM3(BO3)4 with the M = Ga are poorly studied. The LnGa3(BO3)4 3+ 3+ crystals, in particular LnGa3(BO3)4:Tb with the Ln = Y or Gd, are prominent luminescent materials with plasma-discharge conditions, converting a vacuum ultraviolet radiation into a visible light that can be used in high-performance plasma display panels and television devices [13]. In addition, gallium borates can be considered as promising materials not only for luminescent and laser applications, but also for use in spintronics: a large magnetoelectric effect was found in the HoGa3(BO3)4 [14]. 3+ Rare-earth borates LnM3(BO3)4 with the magnetic ions M = Fe or Cr, which are characterized by the presence of two interacting magnetic subsystems (3d- and 4f - ions) in the crystal structure, are even less studied. A long-range antiferromagnetic spin order of the Cr3+ subsystem is observed in the NdCr3(BO3)4 single crystals at TN = 8 K [15]. A phase transition from paramagnetic to antiferromagnetic state was found in the EuCr3(BO3)4 crystals at TN ≈ 9 K [16]. In addition, a magnetic phase transition is also observed in the SmCr3(BO3)4 at Tc = 5 ± 1 K [17]. In a number of rare-earth ferroborates, a significant magnetoelectric effect was found [18,19], which makes it possible to attribute them to a new class of multiferroics [18]. Maximum magnetoelectric and magnetodielectric effects were recorded for the NdFe3(BO3)4 and SmFe3(BO3)4 crystals [19,20]. Such materials can be used as magnetoelectric sensors, memory elements, magnetic switches, spintronics devices, high-speed radiation-resistant MRAM memory, etc. 3+ Rare-earth scandium borates LnM3(BO3)4 with the M = Sc demonstrate an anomalously low luminescence concentration quenching, which is caused by the large distance between the nearest Crystals 2019, 9, 100 3 of 49 Ln ions (~6 Å), and, hence, these compounds are promising high-efficient optical media that can be used in photonics, in particular, to create a diode-pumped compact lasers covering various optical spectral regions [21,22]. Currently, the best materials for medium-power solid-state miniature lasers 3+ are undoped and Cr -doped NdSc3(BO3)4 [22,23]. They are characterized by a high efficiency of laser transitions, on the one hand, and detuning of almost all cross-relaxation transitions, on the other. In addition, the NdSc3(BO3)4 crystals have a high non-linear dielectric susceptibility. Hence, in these crystals, it is possible to convert an infrared radiation of the neodymium laser into a visible one along a certain crystal orientation relative to the propagation of laser radiation. A miniature laser emitted in a green spectral region can be created based on the NdSc3(BO3)4. As can be seen from the short literature review on properties and possible areas of application of the huntite-family borates, these materials are characterized by a combination of different functional properties in one compound. In turn, physical and chemical properties are due to a real composition and crystal structure, influenced by initial composition (the type of rare-earth metal and the nature of M metal), synthesis conditions, and external effects. Determination of structure of huntite-family rare-earth borates becomes very important, since a possible structural transition from one space group to another can be accompanied by a loss (or acquisition) of the center of symmetry and results in an acquisition (or loss) of nonlinear optical and magnetoelectric properties.
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